The production costs for biofuels and certain other bioproducts via microbial fermentation is currently high, particularly compared to oil-derived fuels. Feedstock and feedstock pre-treatment costs for use in such methods can form 50-60% or more of total operating costs. Generally these costs relate to the carbohydrates used as the carbon source in the production of the biofuels. Because these costs are so high, they are one of the primary factors affecting the economic viability of cellulosic and other next generation biofuel manufacturing processes. There is therefore a strong need for lowering these costs and for producing desired products at high yield and high titers. One way to mitigate high feedstock costs is by maximizing feedstock conversion to the product of interest.
However, conventional methods for maximizing feedstock conversion are fraught with difficulties. For example, attempts to ferment gaseous substrates with autotrophic organisms have been hindered by difficulties in reaching suitable concentrations of the substrate and by low titers, which increase isolation-related operating costs. Autotrophic fermentation has also been limited in the range of economically attainable products.
From a metabolic perspective, acetyl-CoA is a central building block and a link between glycolysis and fermentative alcohol production. Consequently acetyl-CoA serves as a focal point for biofuel production in microbial organisms. However, the ability to achieve metabolically efficient production of acetyl-CoA (and high mass yields) has historically been impeded by CO2 loss during decarboxylation reactions involved in classical Embden-Meyerhof-Parnas (EMP) glycolysis. For example, one molecule of glucose (where glucose is the carbon source) under heterotrophic growth conditions may be used to generate two molecules of acetyl-CoA and excess ATP, but this occurs at the “expense” of two CO2 molecules, which are lost in the conversion of pyruvate to acetyl-CoA. In contrast, two molecules of CO2 (where gaseous CO2 is the carbon source) under autotrophic growth conditions may generate one molecule of acetyl-CoA, but this scheme results in a net ATP formation of less than 1, and acetate production (from acetyl-CoA) is required to generate more ATP.
Accordingly, there is a need for fermentation methods and engineering metabolic pathways that minimize—or ideally eliminate—CO2 losses and result in complete conversion of a carbohydrate source into acetyl-CoA without having to sacrifice the acetyl-CoA produced for further generation of ATP.